By virtue of its natural function the respiratory tract is exposed to a slew of airborne pathogens that cause a variety of respiratory ailments. Viral infection of the respiratory tract is the most common cause of infantile hospitalization in the developed world with an estimated 91,000 annual admissions in the US at a cost of $300 M. Human respiratory syncytial virus (RSV) and parainfluenza virus (PIV) are two major agents of respiratory illness; together, they infect the upper and lower respiratory tracts, leading to croup, pneumonia and bronchiolitis (Openshaw, P. J. M. Respir. Res. 3 (Suppl 1), S15-S20 (2002), Easton, A. J., et al., Clin. Microbiol. Rev. 17, 390-412 (2004)). RSV alone infects up to 65% of all babies within the first year of life, and essentially all within the first 2 years. It is a significant cause of morbidity and mortality in the elderly as well. Immunity after RSV infection is neither complete nor lasting, and therefore, repeated infections occur in all age groups. Infants experiencing RSV bronchiolitis are more likely to develop wheezing and asthma later in life. Research for effective treatment and vaccine against RSV has been ongoing for nearly four decades with few successes (Openshaw, P. J. M. Respir. Res. 3 (Suppl 1), S15-S20 (2002), Maggon, K. et al, Rev. Med. Virol. 14, 149-168 (2004)). Currently, no vaccine is clinically approved for either RSV. Strains of both viruses also exist for nonhuman animals such as the cattle, goat, pig and sheep, causing loss to agriculture and the dairy and meat industry (Easton, A. J., et al., Clin. Microbiol. Rev. 17, 390-412 (2004)).
Both RSV contain nonsegmented negative-strand RNA genomes and belong to the Paramyxoviridae family. A number of features of these viruses have contributed to the difficulties of prevention and therapy. The viral genomes mutate at a high rate due to the lack of a replicational proof-reading mechanism of the RNA genomes, presenting a significant challenge in designing a reliable vaccine or antiviral (Sullender, W. M. Clin. Microbiol. Rev. 13, 1-15 (2000)). Promising inhibitors of the RSV fusion protein (F) were abandoned partly because the virus developed resistant mutations that were mapped to the F gene (Razinkov, V, et. al., Antivir. Res. 55, 189-200 (2002), Morton, C. J. et al. Virology 311, 275-288 (2003)). Both viruses associate with cellular proteins, adding to the difficulty of obtaining cell-free viral material for vaccination (Burke, E., et al., Virology 252, 137-148 (1998), Burke, E., et al., J. Virol. 74, 669-675 (2000), Gupta, S., et al., J. Virol. 72, 2655-2662 (1998)). Finally, the immunology of both, and especially that of RSV, is exquisitely complex (Peebles, R. S., Jr., et al., Viral. Immunol. 16, 25-34 (2003), Haynes, L. M., et al., J. Virol. 77, 9831-9844 (2003)). Use of denatured RSV proteins as vaccines leads to “immunopotentiation” or vaccine-enhanced disease (Polack, F. P. et al. J. Exp. Med. 196, 859-865 (2002)). The overall problem is underscored by the recent closure of a number of anti-RSV biopharma programs.
The RSV genome comprises a single strand of negative sense RNA that is 15,222 nucleotides in length and yields eleven major proteins. (Falsey, A. R., and E. E. Walsh, 2000, Clinical Microbiological Reviews 13:371-84.) Two of these proteins, the F (fusion) and G (attachment) glycoproteins, are the major surface proteins and the most important for inducing protective immunity. The SH (small hydrophobic) protein, the M (matrix) protein, and the M2 (22 kDa) protein are associated with the viral envelope but do not induce a protective immune response. The N (major nucleocapsid associated protein), P (phosphoprotein), and L (major polymerase protein) proteins are found associated with virion RNA. The two non-structural proteins, NS1 and NS2, presumably participate in host-virus interaction but are not present in infectious virions.
Human RSV strains have been classified into two major groups, A and B. The G glycoprotein has been shown to be the most divergent among RSV proteins. Variability of the RSV G glycoprotein between and within the two RSV groups is believed to be important to the ability of RSV to cause yearly outbreaks of disease. The G glycoprotein comprises 289-299 amino acids (depending on RSV strain), and has an intracellular, transmembrane, and highly glycosylated stalk structure of 90 kDa, as well as heparin-binding domains. The glycoprotein exists in secreted and membrane-bound forms.
Successful methods of treating RSV infection are currently unavailable (Maggon K and S. Barik, 2004, Reviews in Medical Virology 14:149-68). Infection of the lower respiratory tract with RSV is a self-limiting condition in most cases. No definitive guidelines or criteria exist on how to treat or when to admit or discharge infants and children with the disease. Hypoxia, which can occur in association with RSV infection, can be treated with oxygen via a nasal cannula. Mechanical ventilation for children with respiratory failure, shock, or recurrent apnea can lower mortality. Some physicians prescribe steroids. However, several studies have shown that steroid therapy does not affect the clinical course of infants and children admitted to the hospital with bronchiolitis. Thus corticosteroids, alone or in combination with bronchodilators, may be useless in the management of bronchiolitis in otherwise healthy unventilated patients. In infants and children with underlying cardiopulmonary diseases, such as bronchopulmonary dysphasia and asthma, steroids have also been used.
Ribavirin, a guanosine analogue with antiviral activity, has been used to treat infants and children with RSV bronchiolitis since the mid 1980s, but many studies evaluating its use have shown conflicting results. In most centers, the use of ribavirin is now restricted to immunocompromised patients and to those who are severely ill.
The severity of RSV bronchiolitis has been associated with low serum retinol concentrations, but trials in hospitalized children with RSV bronchiolitis have shown that vitamin A supplementation provides no beneficial effect. Therapeutic trials of 1500 mg/kg intravenous RSV immune globulin or 100 mg/kg inhaled immune globulin for RSV lower-respiratory-tract infection have also failed to show substantial beneficial effects.
In developed countries, the treatment of RSV lower-respiratory-tract infection is generally limited to symptomatic therapy. Antiviral therapy is usually limited to life-threatening situations due to its high cost and to the lack of consensus on efficacy. In developing countries, oxygen is the main therapy (when available), and the only way to lower mortality is through prevention.
RNA interference or “RNAi” is a term initially coined by Fire and co-workers to describe the observation that double-stranded RNA (dsRNA) can block gene expression when it is introduced into worms (Fire et al., Nature 391:806-811, 1998). Short dsRNA directs gene-specific, post-transcriptional silencing in many organisms, including vertebrates, and has provided a new tool for studying gene function. RNAi has been suggested as a method of developing a new class of therapeutic agents. However, to date, these have remained mostly as suggestions with no demonstrate proof that RNAi can be used therapeutically.
Therefore, there is a need for safe and effective vaccines against RSV, especially for infants and children. There is also a need for therapeutic agents and methods for treating RSV infection at all ages and in immuno-compromised individuals. There is also a need for scientific methods to characterize the protective immune response to RSV so that the pathogenesis of the disease can be studied, and screening for therapeutic agents and vaccines can be facilitated. The present invention overcomes previous shortcomings in the art by providing methods and compositions effective for modulating or preventing RSV infection. Specifically, the present invention advances the art by providing iRNA agents that have been shown to reduce RSV levels in vitro and in vivo, as well as being effective against both major subtypes of RSV, and a showing of therapeutic activity of this class of molecules.